Various devices including aircraft and guided missiles which travel at high velocities are controlled by transmitting a signal from a remote station to an infrared (IR) sensor or window located on-board the device. While in operation, the IR sensor or window is exposed to considerable heat loading and erosion due to impact of particles. Such exposure may exceed the working capabilities of the IR sensor or window. Even the smallest atmospheric dust particles can scratch the IR sensor or window, which over time can cause considerable erosion effects on the optical transmissivity of the IR sensors or windows. The term “optical transmissivity”, as used herein, refers to the ability a material to allow desired wavelengths of electromagnetic radiation to pass through it.
Materials which can be used to make IR sensors or windows include, but are not limited to: zinc sulfide (ZnS), zinc selenide (ZnSe), germanium (Ge), silicon (Si), gallium arsenide (GaAs), gallium phosphide (GaP), fused silica, aluminum oxynitride (AlON), sapphire (Al2O3), magnesium oxide (MgO), spinel (MgO—Al2O3), cubic zirconia (c-ZrO2), lanthanum-doped yttria, yttria (Y2O3), mixed fluoride glasses, chalcogenide glasses and other optical transmissive materials. These optical transmissive materials can be temperature sensitive (i.e., they have a low softening temperature) that can fail due to thermal shock caused by atmospheric friction at high velocities. Additionally, optical transmissive materials are generally soft materials and therefore damage easily upon use.
Various coatings are required to enhance the performance of optical elements used in infrared imaging systems. Infrared devices typically operate in the 3-5 and 8-12 micron regions due to the lack of absorption of IR energy by the atmosphere at those wavelengths. However, where a material used for IR coatings is generally comprised of C—H and/or N—H bonds, such material will absorb in the 3-5 and 8-12 micron range. Coatings and films are typically applied to provide antireflective properties and protect optical transmissive materials from damage caused by thermal shock, abrasion, or erosion. One coating that has been successfully employed in protecting optical transmissive materials is a hard carbon film that has diamond-like properties, e.g., a diamond-like carbon (DLC) film. However, DLC films often require high-temperatures and atomic hydrogen for deposition, both of which can degrade the optical transmissive material where various interlayers are not employed. Where suitable interlayers have been employed, such interlayers may delaminate at high-temperatures, further complicating the process.
Furthermore, interlayers and DLC coatings may reduce the level of optical transmissivity required for such devices to a level below an acceptable threshold. As indicated above, the C—H bond is absorptive of energy in the range in which infrared transmissions occur. Thus, a useful protective coating or interlayer for use with optical transmissive materials must itself be acceptably optically transmissive. The optical transmissivity of the coating or interlayer itself must also be able to withstand high-operating temperatures.